1877.] 



and Steel during Heating and Cooling. 



131 



cooling the finger passes beyond zero, indicating now that the wire is 

 permanently shortened, whereas in the primary heating it was permanently 

 elongated. 



A comparison of these two series of experiments will show us that, 

 during the heating of these wires, there is going on at the same time a 

 whole series of phenomena, some of which, although different in nature, 

 are alike as to result, while others are different both as to nature and 

 result. We have, in the first place, the ordinary expansion, which we 

 will designate the " dynamical expansion." In the primary heating an 

 amount of expansion will be seen to occur which has no equivalent in the 

 dynamical contraction of cooling. The secondary heatings reveal to us 

 the existence of a contraction, which is also excited during the heating, 

 and the action of which is to limit the dynamical expansion previously 

 referred to. That it masks a portion of the dynamical expansion is ren- 

 dered obvious by the fact that, in cooling, the finger moves beyond zero, 

 showing that the wire has contracted more in cooling than it expanded in 

 heating. 



In addition to this, we have also the phenomena of the kicks them- 

 selves ; and these may be described as consisting of a temporary contrac- 

 tion and expansion. 



During the heating of a wire, when in its commercial state, there is one 

 expansive and two contractive tendencies exerting their powers at one and 

 the same time ; and conversely, in the act of cooling, there is one contrac- 

 tive and two expansive influences at work, viz. : — ordinary cooling con- 

 traction; an expansion which is the opposite of the temporary contraction 

 of heating — the cooling-hick ; and an expansion which is the opposite in 

 nature, but not necessarily in amount, of the contraction of heating — 

 crystalline expansion. In every case the interpretation of the general 

 result depends upon the accurate estimate of the extent to which these 

 interacting forces have modified each other's effects. Thus during the 

 heating we have a temporary and a permanent contraction, assisting each 

 other to oppose the dynamical expansion. In cooling, on the other hand, 

 we have two expansions, one of which is temporary and the other perma- 

 nent, opposing the dynamical contraction of cooling. 



It will be desirable to carefully define the nature of these respective 

 influences, and the terms which will be used to distinguish them in this 

 research. 



I. Dynamical Expansion. — This term will be used to distinguish the 

 ordinary dilatation produced in bodies generally, by raising them from a 

 given temperature to a higher one, and which is exactly counterbalanced 

 by contraction when the original temperature is regained. It is proposed 

 to call this kind of contraction " the dynamical contraction." 



II. Contraction of Heating. — rn the diagrams representing second 

 heatings [vide fig. 2] we get the first inkling of the existence of a con- 

 tractive or shortening influence excited by heat simultaneously with the 



